"This is covert surveillance. It is very hard for any of us to have trust in him as the senior law enforcer... in this country.

"He has some serious explaining to do," she said.

The call to the attorney general - the chief legal adviser to the government - was about the admissibility of wire tap evidence in court, but it did not concern a particular case.

Sir Ian, who is thought to be on holiday, has been criticised previously for his handling of the aftermath of the shooting of Brazilian Jean Charles de Menezes by anti-terror police at Stockwell Tube station last July.

Remarks he made about the Soham murders also generated a furore in the media.

Sir Ian has also admitted taping calls with senior officials from the Independent Police Complaints Commission.

Surprise at recording

An IPCC spokesman said the taped conversations came to light as part of its inquiry into the aftermath of the shooting.

"We understand three telephone conversations between senior IPCC personnel and the commissioner of the Metropolitan Police have been recorded without our prior consent," said the spokesman.

"One of these conversations was between our chair Nick Hardwick and the commissioner.

"We are surprised about the recording of calls and now have the recordings. We are dealing with this issue."

Consent issue

The spokesman added that no formal investigation into the allegations was taking place.

The recording and monitoring of telephone calls is governed by a number of different pieces of UK legislation, according to Ofcom, the telephone industry regulator.

It is not illegal for individuals to tape conversations providing the recording is for their own use, under the Regulation of Investigatory Powers Act 2000 (RIPA).

It is a civil, not criminal, matter if a conversation or e-mail has been recorded and shared unlawfully.

If a person intends to make the conversation available to a third party, they must first obtain the consent of the person being recorded.